1. Field of the Invention
The present invention generally relates to electronic phase shifting circuits, and more specifically to a line-loop diode phase bit circuit suitable for use as a 180.degree. phase bit for an antenna element in a phased radar antenna array.
2. Description of the Related Art
Phased array radar systems include a large number, typically thousands, of antenna elements, each of which radiates an electromagnetic signal that, by constructively adding in space, forms an electromagnetic beam which is caused to spatially sweep or scan by varying the phase of each of the individual antenna element signals. Since a separate phase shifting circuit is required for each antenna element, it is important in practical application to minimize the size, complexity and cost of these components.
Phase shifting circuits for radar systems are generally digital, including several delay elements or phase bits connected in cascade, each of which is electronically switchable to provide either a zero phase shift or a predetermined phase shift. A four bit phase shifting circuit, for example, includes four phase bits which provide phase shifts of 180.degree., 90.degree., 45.degree. and 22.5.degree. respectively. Switching the phase bits in the proper combinations enables the phase shifting circuit to produce a phase shift which is progressively variable in 22.5.degree. increments from 0.degree. to 337.5.degree..
As described in a textbook entitled "RADAR HANDBOOK Second Edition", by M. Skolnik, McGraw-Hill, 1990, pp. 7.63-7.67, diode phase bit circuits, to which the present invention relates, are conventionally based on a switched-line, hybrid-coupled or loaded-line configuration.
The switched-line phase bit circuit includes a pair of single-pole, double-throw switches for switching one of two lengths of transmission line into a circuit. This circuit generally requires four diodes. Phase shift is obtained by switching between one line which constitutes a reference path and a second line which provides a delay path.
The hybrid-coupled phase bit circuit includes a 3-decibel hybrid coupler with a pair of balanced diode switches connected to identical split arms of the hybrid. The hybrid-coupled bit is used extensively because it enables large phase shifts while using only two diodes.
The loaded-line phase-bit circuit includes a number of pairs of switched susceptances spaced at one-quarter wavelength intervals along a transmission line. Phase shift is obtained by switching the susceptances between an inductive and a capacitive state. Phase shift for this circuit is limited to about 45.degree. for one pair of diodes.
Generally, the larger the phase shift required, the more difficult the phase bit circuit is to design. In order to reduce cost, the number of diodes used must be minimized. The switched-line phase bit circuit requires four diodes, and is limited in power handling capability since it requires small, low loss diodes in order to provide low insertion loss. The loaded-line phase bit is capable of high power operation, but is too large and bulky for many applications.
Phase shifting circuits may be fabricated at low cost using a microstrip construction and a thick film printing technique where the circuit metallization, capacitors and resistors are formed on an alumina substrate with multiple printing and firings. The 3-decibel "Lange" hybrid coupler (an interdigitated structure), which is the primary element in current hybrid-coupled phase bit circuits, is a complicated and expensive element to fabricate due to close line width tolerances, minimum line width, and inclusion of critical gaps and line separations. These constraints preclude the fabrication of Lange couplers using current thick film printing technology.